DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

users of ERP and CRM vendor solutions and ported applications with not well known growth,
and can lead to application outages.
Default PRIQTY and SECQTY values and often user provided PRIQTY and SECQTY values
are too small, resulting in track allocation rather than cylinder allocation. As a result the
performance of SQL SELECT and INSERT processing can be penalized by 2x or more.
The first objective of this enhancement is to avoid the situation where a DB2 managed page
set reaches the VSAM maximum extent limit of 255 before it can reach the maximum data set
size which may be less than 1 GB or 1 GB, 2 GB, 4 GB, 8 GB, 16 GB, 32 GB or 64 GB.
The second objective of the enhancement is to remove the need for a user to specify the
secondary quantity for a DB2 managed page set, or even both the primary and secondary
quantities, either when creating it initially or by using an ALTER statement later. The overall
goal is to provide a solution which would benefit all users of DB2 in terms of improved DBA
productivity and preventing application outages.
DB2 V8 adds a new DSNZPARM parameter called MGEXTSZ (with a global scope) which
enables the use of a “sliding secondary space allocation” for DB2 managed data sets
(STOGROUP defined). The values are: NO and YES. The default value for MGEXTSZ is NO.
The enhancement will help to reduce the number of out of space conditions, eliminate need
for PRIQTY and SECQTY specification on SQL CREATE, improve user productivity, and
avoid the performance penalty associated with small extent sizes.
This patented solution should benefit all users of DB2, not just for ERP and CRM solutions,
ported and new applications. It delivers autonomic selection of data set extent sizes with a
goal of preventing out of extent errors before reaching maximum data set size.
An increasing secondary quantity size up to 127 extents is allocated and a constant number
of 559 cylinders for 32 GB and 64 GB data sets is used thereafter. Figure 5-6 shows the
sliding scale mechanism for 64 GB data sets with 255 extents. It assumes an initial extent of 1
cylinder.
Extent Size (CYLS)
600
500
400
300
200
100
0
16
Sliding scale for 64 GB data sets
32
Figure 5-6 Sliding scale for 64 GB data sets
64
96
128 255
Extent Number
Chapter 5. Availability and capacity enhancements 227